This invention relates to the storage and transportation of compressed gases. In particular, the present invention includes methods and apparatus for storing and transporting compressed gas, methods and apparatus for construction of gas storage systems, land vehicles for transporting the compressed gas and storage components for the gas, methods for loading and unloading the gas from those systems, and methods for utilizing gas storage systems. More particularly, the present invention relates to a compressed natural gas storage system specifically optimized and configured to a gas of a particular composition.
The need for transportation and storage of gas has increased as gas resources have been established around the globe. Traditionally, only a few methods have proved viable in transporting and storing gas in large quantities. One transportation method is to build a pipeline and “pipe” the gas directly to a desired location. A typical storage method is to simply build large pressure vessels or storage tanks to store the gas at ambient conditions or at a slightly pressurized condition. As an alternative to large pressure vessels pipeline loops have also been constructed to store a quantity of gas at pipeline conditions.
Due to the limitations of ambient, or near-ambient, storage and transportation methods, other methods have emerged. The most readily apparent problem with gas storage and transportation is that in the gas phase, even below ambient temperature, a small amount of gas occupies a large amount of space. Storing material at that volume is often not economically feasible. The answer lies in reducing the space that the gas occupies. Initially, it would seem intuitive that condensing the gas to a liquid is the most logical solution. A typical natural gas (approximately 90% CH4), can be reduced to 1/600th of its gaseous volume when it is compressed to a liquid. Gaseous hydrocarbons that are in the liquid state are known in the art as liquefied natural gas, more commonly known as LNG.
As indicated by the name, LNG involves liquefaction of the natural gas and normally includes transportation and storage of the natural gas in the liquid phase. Although liquefaction would seem a solution to the storage and transportation problems, the drawbacks quickly become apparent. First, in order to liquefy natural gas, it must be cooled to approximately −260° F., at atmospheric pressure, before it will liquefy. Second, LNG tends to warm during long term storage and transport and therefore will not stay at that low temperature so as to remain in the liquefied state. Cryogenic methods must be used in order to keep the LNG at the proper temperature during transport. Thus, the cargo containment systems used to transport LNG must be truly cryogenic. Third, the LNG must be re-gasified at its destination before it can be used.
Cryogenic process requires a large initial cost for LNG facilities at both the loading and unloading ports. The containment systems and storage vessels require exotic metals to hold LNG at −260° F. Liquefied natural gas can also be stored at higher temperatures than −260° F. by raising the pressure but, unless temperatures are kept relatively low, the efficiency of the storage system will quickly deteriorate. Therefore, although the storage temperature may be above −260° F., cryogenic problems still remain and the containment systems now must be pressure vessels. This may not be an economical alternative.
In response to the technical problems of ambient condition storage and transportation and the extreme costs and temperatures of LNG, the method of transporting natural gas in a compressed state was developed. The natural gas is compressed or pressurized to higher pressures, which may be chilled to lower than ambient temperatures, but without reaching the liquid phase. This is what is commonly referred to as compressed natural gas, or CNG.
Several methods have been proposed that are related to the storage and transportation of compressed gases, such as natural gas, in pressurized vessels by overland carriers. The gas is typically transported and stored at high pressure and low temperature to maximize the amount of gas contained in each gas storage system. For example, the compressed gas may be in a dense single-fluid (“supercritical”) state, that is characterized by the presence of a very dense gas but with no liquids.
The transportation of CNG by overland vehicles typically employs trucks or trains. The vehicles include gas storage containers, such as metal pressure bottle containers. These storage containers are resistant internally to the high pressure and low temperature conditions under which the CNG is stored. The containers must be internally insulated throughout to keep the CNG and its storage containers at approximately the loading temperature throughout the travel and delivery of the gas and also to keep the substantially empty containers near that temperature during the return trip.
Before the CNG is transported, it is first brought to the desired operating state, normally by compressing the gas to a high pressure and cooling it to a low temperature. For example, U.S. Pat. No. 3,232,725, hereby incorporated herein by reference for all purposes, discloses the preparation of natural gas to conditions suitable for large volume marine transportation. After compression and cooling, the CNG is loaded into the storage containers of the storage systems. The CNG is then transported to its destination.
When reaching its destination, the CNG is unloaded, typically at a terminal comprising a number of high pressure storage containers or an inlet to a high pressure turbine. If the terminal is at a pressure of, for example, 1000 pounds per square inch (“psi”) and the storage containers are at 2000 psi, valves may be opened and the gas expanded into the terminal until the pressure in the storage containers drops to some final pressure between 2000 psi and 1000 psi. If the volume of the terminal is very much larger than the combined volume of all the storage containers together, the final pressure will be about 1000 psi.
Using conventional procedures, the transported CNG remaining in the storage containers (the “residual gas”) is then compressed into the terminal storage container using compressors. Compressors are expensive and increase the capital cost of the unloading facilities. Additionally, the temperature of the residual gas is increased by the heat of compression. The higher temperature increases the required storage volume unless the heat is removed, or excess gas removed, and raises the overall cost of transporting the CNG.
Previous efforts to reduce the expense and complexity of unloading CNG, and the residual gas in particular, have introduced problems of their own. For example, U.S. Pat. No. 2,972,873, hereby incorporated herein by reference for all purposes, discloses heating the residual gas to increase its pressure, thereby driving it out of the vehicle storage containers. Such a scheme simply replaces the additional operating cost associated with operating the compressors with an operating cost for supplying heat to the storage containers and residual gas. Further, the design of the piping and valve arrangements for such a system is necessarily extremely complex because the system must accommodate the introduction of heating devices or heating elements into the storage containers.
In summary, although CNG transportation and storage reduces the capital costs associated with LNG, the costs are still high due to a lack of efficiency by the methods and apparatus used. This is due primarily to the fact that prior art methods do not optimize the vessels and facilities for a particular gas composition. In particular, prior art apparatus and methods are not designed based upon a specific composition of gas to determine the optimum storage conditions for that particular gas.
U.S. Pat. No. 4,846,088 discloses the use of pipe for compressed gas storage on an open barge. The storage components are strictly confined to be on or above the deck of the ship. Compressors are used to load and off load the compressed gas. However, there is no consideration of a pipe design factor and no attempt is made to utilize the maximum compressibility factor for the gas.
U.S. Pat. No. 3,232,725 does not contemplate a specific compressibility factor to determine the appropriate pressure for the gas. Instead, the '725 patent discloses a broad range or band to get greater compressibility. However, to do that, the gas container wall thickness will be much greater than is necessary. This would be particularly true when operated at a lower pressure causing the pipe to be over designed (unnecessarily thick). The '725 patent shows a phase diagram for a mixture of methane and other hydrocarbons. The diagram shows an envelop inside which the mixture exists as both a liquid and a gas. At pressures above this envelop the mixture exists as a single phase, known as the dense phase or critical state. If the gas is pressured up within that state, liquids will not fall out of the gas. Also, good compression ratios are achieved in that range. Thus, the '725 patent recommends operation in that range. The '725 patent does not pick a particular gas composition to match a particular gas reservoir.
The '725 patent graph is based on the lowering of temperatures. However, the '725 patent does not design its method and apparatus by optimizing the compressibility factor at a certain temperature and pressure and then calculating the wall thickness for the storage container needed for a certain gas. Since much of the capital cost comes from the large amount of metal, or other material, required for the pipe storage components, the '725 misses the mark. The range offered in the '725 patent is very broad and is designed to cover more than one particular gas mixture, i.e., gas mixtures with different compositions.
U.S. Pat. No. 4,446,232 discloses offloading using a displacing fluid. The '232 patent does not consider low temperature fluids. It also does not consider onshore storage and thermal shock. The '332 patent carries the displacement fluid on the vessel which is used to displace sequential tanks. No mention is made of low temperature requirements.
U.S. Pat. No. 5,429,268 discloses the storage of compressed natural gas in pipes, which may be stationary or mobile as required. The pipes are supported in a vessel cradle having semi-circular concave portions.
U.S. Pat. No. 5,566,712 discloses a system for handling, storing, transporting, and dispensing cryogenic fluids, liquid natural gas, and compressed natural gas. The system includes a container in a frame disposed on a flat car. The gas may be injected into the engine's combustion chamber.
Another problem in the energy industry relates to gas storage and occurs during “peak shaving.” Energy consumption by consumers is not constant over time and there are periods when there is a greater demand for energy than other periods, particularly during the work day when energy consumption is higher due to industry and business operations and particularly when the temperature during the day is at its highest requiring additional energy due to the widespread operation of air conditioning. Peak shaving occurs when a power company encounters a time period when there is a peak demand for energy or power. That spike in energy consumption is met by consuming additional gas to generate the additional energy to meet that spike demand. Presently, power companies pay for a steady delivery of gas throughout the day at a volume which will meet peak shaving even though such gas volume is not required throughout the day. Thus power companies pay for this excess capacity without regard to peak periods of demand which is expensive. For example power companies pay the pipeline companies for this peak capacity throughout the whole heating season. It would be an advantage if the power companies could draw upon a reserve of gas during peak shaving to avoid paying for excess capacity of gas to produce additional energy during peak demand periods.
Another concern associated with natural gas relates to the development and testing of new oil and gas wells, particularly off-shore wells. Gas is typically produced during a test of the new well. Presently when conducting an extended well test on a new offshore well, a production package is disposed on the off-shore rig to separate the oil from the gas being produced. Although the government has a policy of not allowing the flaring of gas, the government has been allowing the gas produced by the hew well to be flared into the atmosphere. Of course, it is not cost effective to run a pipeline to the rig for the gas until the well has been tested to ensure enough gas is being produced to warrant a pipeline. An alternative to flaring the gas is needed.
The present invention overcomes the deficiencies of the prior art by providing a method for optimizing a storage container for compressed gas and a method for loading and unloading the gas.